Bias-driven terahertz negative conductivity and transmission enhancement

Bias-driven terahertz (THz) negative conductivity and transmission enhancement based on structures composed of monolayer graphene, MXene (Ti3C2Tx) and poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate):dimethyl sulfoxide (PEDOT:PSS:DMSO) caused by photo-excited or bias-driven are investigated. The THz signals that pass through the monolayer graphene/quartz, MXene/quartz, thinner and thicker PEDOT:PSS:DMSO/quartz structures are enhanced effectively under photo excitation or application of an external bias voltage in a THz time-domain spectroscopy (THz-TDS) system. This enhancement of THz transmission indicates that negative conductivity occurs, which is consistent with the results based on the monolayer graphene, thinner PEDOT:PSS:DMSO samples detected using an optical pump-THz probe (OPTP) system. The conductivity reduction mechanism is explained using an established Drude model. After photo excitation, this results in the generation of photo-induced carriers and an increase in the average scattering rate of the carriers. And as a consequence of the energy injection into the carrier system of the sample after applying a bias, the carrier temperature increases rapidly; this results in the generation of carriers and an increase in the average scattering rate of the carriers. However, the increase in the average scattering rate of the carriers is much higher than the increase in the average carrier density, which leads to the negative conductivity and the increased THz transmission.